The present invention relates generally to liquid crystal display (LCD) devices and, more particularly, to transreflective LCD devices, also known as “semi-transmissive” LCDs.
LCD devices are nonluminous display panels which visually display on-screen images by adjustment of amounts of transmitted light rays, unlike self-luminous display devices, such as cathode ray tube (CRT) monitors and plasma display panels (PDPs). LCD panels have advantageous features, such as slimness, light weight, and low power consumption.
LCD devices typically include transmissive LCD panels and reflective LCD panels, wherein the former displays images through adjustment of the amount of transmission light emitted from a light source on a back plane, called the back-light, whereas the latter displays images by adjustment of the amount of reflection rays of the incoming outside light, e.g., room illumination light, sunlight, etc., while letting the light reflect from a front surface side. Recently, a hybrid type of LCD device is known, called the transreflective or “semi-transmissive” LCD panel. This type of LCD panel has image display functionalities of both the transmissive and the reflective LCDs-that is, it is usable as a reflective LCD in bright environments and is operable as a transmissive LCD in dark environments. Letting its backlight turn off in bright environments contributes to a decrease in power consumption. Turning the backlight on in dark environments provides users with enhanced viewability. The transreflective LCD panel is preferably adaptable for use in portable or handheld electronics tools, such as mobile cellular phones, digital cameras or the like, which are expected to be used under various kinds of illumination conditions.
In currently available transreflective LCD panels, there are several display technologies, such as electrically controlled birefringence (ECB) technique with the initial orientation of liquid crystal (LC) material being almost in parallel with a substrate surface, twisted nematic (TN) technique, and vertical alignment (VA) scheme with LC material being oriented in a direction substantially perpendicular to the substrate surface. In the case of VA display, a phase difference in the direction at right angles to the substrate surface becomes nearly zero in the initial orientation state because of the fact that LC material is oriented vertically to the substrate. Thus, an increased gap margin is attainable while at the same time enhancing reflection contrast ratios.
JP-A-2000-187220 discloses therein optical designs of a transreflective VA-LCD panel. This Japanese patent bulletin teaches an approach to optimally designing the retardation of each of reflection and transmission regions. To do this, a step-like surface difference is provided at the reflection region to cause LC layer to be variable in thickness so that the thickness of a layer portion at the reflection region is almost one-half of the thickness of an LC layer portion at the transmission region. In addition, in order to make the transmission and reflection areas equal to each other in optical characteristics, a quarter wavelength (λ/4) plate is disposed outside of a respective one of the upper and lower panel substrates. This λ/4 plate is laid out to cover both the transmission and reflection regions.
Unfortunately, suggested advantages of the LCD panel structure as taught from JP-A-2000-187220 do not come without accompanying a penalty which follows. The LC layer experiences unwanted entry of circularly polarized light due to the presence of the λ/4 plates disposed external to the upper and lower substrates. This poses a problem as to a decrease in transmission contrast ratio due to occurrence of light leakage during black displaying in cases where there are deviations of the optical axes of λ/4 plates and/or variations of in-plane phase difference.